Liquid development apparatus for electrophotography

ABSTRACT

An electrophotographic developing apparatus comprising: multiple carrier rollers, held in supporting means, conveying an electrophotographic sheet material to be processed and driven at a common speed by driving means; two endless belts hung at the side ends of said carrier rollers to form a loop including at least some of said carrier rollers; multiple development electrodes arranged facing to carrier rollers; and a means of supplying liquid developer onto the electrophotographic sheet which is conveyed between said carrier rollers and said development electrodes, whereby a developer pool is formed on the electrophotographic sheet, said pool being limited by the sheet as bottom plane, and the adjacent developing electrodes and the supporting means as side borders, said sheet being conveyed with its rear surface in contact with said endless belts.

United States Patent [191.

Fukushima et a1.

[ Jan. 8, 1974 1 LIQUID DEVELOPMENT APPARATUS FOR ELECTROPHOTOGRAPHY[73] Assignee: Fuji Photo Film Co., Ltd.,

Kanagawa, Japan 221 Filed: Dec. 30, 1971 211 App]. No.: 214,252

[30] Foreign Application Priority Data Dec. 30, 1970 Japan 45/122799[52] US. Cl 118/637, 95/89A, 117/37 LE, 11'8/407, l18/DlG. 23, 355/10[51] Int. Cl .f. G03g 13/10 [58] Field of Search 118/410, 419, 637,

ll8/DIG. 23, 625, 407, 424; 117/37 LE, 93.4 A; 355/10; 95/89 R 3,643,6282/1972 Sugarman 118/637 3,299,791 l/1967 Fukuda 95/89 3,650,622 3/1972Morse 355/10 3,672,329 Yamanoi 118/637 Primary ExaminerMervin SteinAssistant Examiner-Leo Millstein Attorney-Gerald J. Ferguson, Jr. et a1.

[57] ABSTRACT An electrophotographic developing apparatus comprising:multiple carrier rollers, held in supporting means, conveying anelectrophotog raphic sheet material to be processed and driven at acommon speed by driving means; two endless belts hung at the side endsof said carrier rollers to form a loop including at least some of saidcarrier rollers; multiple development electrodes arranged facing tocarrier rollers; and a means of supplying liquid developer onto theelectrophotographic sheet which is conveyed between said carrier rollersand said development electrodes, whereby a developer pool is formed onthe electrophotographic sheet, said pool being limited by the sheetasbottom plane, and the adjacent developing electrodes and the supportingmeans as side borders, said sheet being conveyed with its rear surfacein contact with said endless belts.

2 Claims, 7 Drawing Figures PAIENTEIJJAII 8:914

SHEEI 1 [If 5 PAIENTED 8 SHEET 2 0F 5 FIG. 2

FIG. 4

PATENTED 81974 3. 783 827 SHiU 3 OF 5 FIG.3

PATENTEB JAN 8 i974 SHEET [1F 5 PAIENTEDJ 81974 3,783,827

sum 5 0r 5 FIG. 60

LIQUID DEVELOPMENT APPARATUS FOR ELECTROPIIOTOGRAPI-IY BACKGROUND OF THEINVENTION x 1. Field of the Invention This invention relates to adevelopment apparatus for use in electrophotography and, inparticular,to one suited to process flexible electrophotographic sheet materials toobtain visible images.

2. Description of the Prior Art A flexible electrophotographic sheetmaterial may be produced by treating a flexible support such as paper ora plastic film to impart electroconductivity thereto and providingthereon a photoconductive layer by vacuum deposition of selenium or bycoating thereon a mixture comprising an inorganic photoconductor such aszinc oxide or titanium oxide and a resinous binder. The sheet materialthus prepared is uniformly charged under subdued light to sensitize thephotoconductive layer, and then image-exposed whereby the surface chargedecays in proportion to the intensity of impinged light and acorresponding electrostatic latent image results. Then the sheet isbrought into contact lines of force, however, above the photoconductivelayer do not distribute in proportion to the charge density of thelatent image, which gives rise to the so-called edge effect in thedeveloped image-Hence the resulting image takes an appearance quitedifferent from the original one. To resolve this problem it is a commonpractice to employ grounded conductor, which is referred to as adevelopment electrode, which is located in parallel relation-to andclose to the latent image bearing surface and functions to reduce theedge development. ln this case the resulting toner image becomes a farmore exact reproduction of the charge distribution of the latent image.This is attributed to the fact that the conductive development electrodeincreases the number of lines of force directed above the surface fromthe latent image and effectively absorbs them. Such an effect isreferred to as the development electrode effect. This effect is moremarked with a smaller specific resistance electrode and with a smallergap between the electrode and the surface to be developed.

In the case of electrophoretic development, however, too small anelectrode gap disturbs the supply of developerliquid onto the latentimage, sometimes causing a final image with a low saturation density orone with mottles or non-uniformity. Hence, a'liquid developmentapparatus is required which exhibits an intense electrode effect and atthe same time permits a sufficient amount of developer to reach thedeveloped surface.

In addition to the edge effect, streaks and nonuniformity caused by aninsufficient developer supply are the main defects to be'considered.When high and low density areas exist contiguously, streaks are observedwhich flow out of the former to the latter area like the tail of acomet. This phenomenon has been explained as follows. When a relativemovement exists between the developer and the latent image bearingsurface, toner particles coming upon a high charge density area aresubject to an electrostatic attraction as well as inertia due todeveloper flow, which leads to toner deposition diffusing out of thehigh density area towards the downstream side into the low density area.As is empirically known, a larger velocity of the relative movement anda greater electrode effect for a constant relative velocity gives morestreaks.

Another serious problem attending liquid development is the difficultywith which the developer is supplied uniformly on the whole surface ofthe sheet to be developed. Even when a uniform supply is realized,uniform development cannot be expected if the amount of the developer isnot abundant. Thus the developer has to be supplied uniformly andsufficiently in a relatively brief period, since prolonged developmentwill bring about a decay of the latent image charge and at the same timean increase of fog due to the residual potential. I Y

The decay of the latent image charge is effected, besides through theneutralization of the charge by toner deposition, by the neutralizationthereof .by ionic species other than the toner, and also by leakagethrough the photoconductive coating. Such charge decay provides adeveloped image with a lower saturation toner density than what would beexpected for perfect neutralization of the charge solely by tonerdeposition.

Residual charge is observed at thehighlight regions in an exposedcoating where a sufficient amount of light has been irradiated and notoner deposition is expected. At the early stage of development, tonerdeposition proceeds at high charge density areas, and then progressivelyat lower areas. When the development is extended, which is also notpreferred from the viewpoint of processing efficiency, the residualcharge begins to be detected, thus forming an undesirably high level ofbackground.

From the above descriptions, one will understand, especially in the caseof developingcontinuous-tone images, the significance of a developmentmethod which can provide, in a brief developing'period, toner imagesfree of edge effect, streaks, and mottles due to non-uniform developersupply, and which does .not have significant residual charge, thusgiving a low back ground. As is obvious from .the above explanations,conditions conflicting each other-must be fulfilled in order to solveall of these problems. To minimizethe edge effect leads to an extremelysmall'electrode gap between the electrode and the surface to bedeveloped. Such a small gap hinders the ample supply and exchange ofdeveloper, resulting in lack of uniformity in the extent of developmentthroughout the whole surface. If one resorts to a swift stream of thedeveloper, streaks tend to appear due to a large relative velocitybetween the latent image bearing surface and the de- SUMMARY OF THEINVENTION The principal object of the present invention is to disclosedeveloping apparatus fulfilling the abovedescribed mutually conflictingrequirements as much as possible with simple and compact structures.

Another object of the invention is to disclose apparatus suited fordeveloping continuous-tone images on flexible electrophotographic sheetmaterials.

Apparatus in accordance with the present invention can develop imagesshowing a high degree of uniformity in the extent of development, andfree of streaks which are frequently observed at the boundaries betweenvery highand low density areas. Moreover, it can easily be incorporatedin automatic processing machines because of its simple and very compactstructure.

The essential feature of the invention lies in providing a temporarypool of developer on the surface to be developed only when anelectrophotographic sheet is fed into the apparatus. Such a developerpool enables refreshment and supply of developer without causingrelative movement of developer to the image surface. The word pool inthe present specification means a state wherein a developer layer notthinner than two mms is stationarily formed on the surface to bedeveloped and disturbance occurring near the top of said layer is notdirectly propagated onto the image surface.

Care must be taken, as a matter of course, in the design of theapparatus so that there exists between the liquid layer and the imagesurface no relative large difference of the flowing rates of 3 cm/sec.

Developer supply by means of pool formation is proved to be quite evenand thus provides uniformly developed images. Further, the developerspraying from supply nozzles does not directly run against the imagesurface but onto the upper surface of the pool, so that the relativevelocity of the developer to the image surface is small enough to avoidthe occurrence of streaks.

A flat conductive plate may be used as the development electrode, whichis immersed in the developer pool facing the image surface. However,such an arrangement tends to cause uneven development due toinsufficient supply of developer, since the electrode covers the surfacewith a small spacing. According to our experiments, as a more preferredarrangement, an electrode comprising multiple conductive rollers ofsmall diameter is employed, in which case the developer liquid isprovided through the spacing between the rollers onto the image surface.As the rollers are rotated at the same circumferential speed as thetransport speed of the sheet, the relative movement between thedeveloper and the image surface is minimized, thus preventing streaks.

In the apparatus in accordance with the present invention, the developerpool is formed only when an electrophotographic sheet is fed into theapparatus, and after the sheet is exhausted the liquid falls down andcirculates freely. Therefore, refreshment of the developer is completedfor every newlycoming sheet.

In a development method in which a developer pool is always formed in avessel through which sheet materials are passed, the circulation orexchange of developer is inferior to the present method.

BRIEF DESCRIPTION or THE DRAWINGS FIG. 1 illustrates a cross-sectionalside view of one embodiment of the invention, while FIG. 2 shows thecrosssectional view of the same embodiment cut along the line A-A' inFIG. 1;

FIG. 3 illustrates another embodiment of the invention by thecross-sectional side view, which differs from the foregoing embodimentin the use of another pair of endless. belts of narrow width 25suspended between the electrode rollers at the side ends. These beltshold the side edges of the sheetwhich are outside the imaging areawhereby the electrode rollers are lifted up by the thickness of saidbelts 25 so as to leave an electrode gap of the same magnitude.

As is clearly seen from FIG. 4, which is the crosssectional longitudinalview of the apparatus shown in FIG. 3, there remains a uniform gapbetween the sheet surface and the electrode rollers since the endlessbelts 25 hold only the side edges of the sheet.

FIG. 5 illustrates still another preferable embodiment of the inventionin the form of schematic crosssectional side view. This embodimentdiffers from that shown in FIG. 3 in that the endless belt 15 is nothung over the whole carrier rollers. Instead, the first and terminalones are excluded of the loop.

Other types of auxiliary electrode are shown in FIG. 6; in FIG. 6a,aflat plate is used as electrode, while in FIG. 6b, one having a crosssection like that of a trough is employed into which the developer isfed. This electrode proved quite effective to reduce the nonuniformityof developer supply.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention willnow be explained in more detail by describing some practical embodimentswith reference to the drawings.

Referring now to FIG. 1, an electrophotographic sheet is coming into thedeveloping unit with the surface upward on which is formed anelectrostatic latent image. Carrier rollers 1-7 have their axes fixed atthe bearings provided in side supporting boards (not shown) and aredriven by an external motor (not shown) through gears or chains (notshown) at a constant and common circumferential speed. Electrode rollers8-14 are made of a conductive material, such as metal. On both side tipsof the carrier rollers are suspended two endless belts 15. Developer isfed through supply nozzles 16-21 and is kept in a reservoir tank 22,circulated through an exit 23 provided in the bottom of the tank to thenozzles by means of pump (not shown). When the toner is consumed and itsconcentration is reduced after processing of a large number of sheets, aseparately kept replenishing concentrate is added to the developerautomatically or manually.

Developer is sprayed from nozzles 16-21 through fine slits or lines ofsmall openings. Guide plates 22-27 are provided for each of the nozzlesso as to ensure a uniform flow of developer. The guide plates and therollers form a wedge-shaped space, in which the liquid is briefly heldup instead of directly running against the sheet; this stagnation iseffective to reduce the occurrence of streaks and provide an even supplyof developer.

The essential feature of the invention, however, lies in the use of theendless belts 15. As will now be explained referring to FIG. 2, thebelts are hung at the side tips of the carrier rollers and rotated incontact engagement with both side support boards 28 and 29. The beltsprevent the developer from falling freely through the gap between thesheet and the side boards 28 and 29. Since the sheet to be processed 100places its side ends on the belts 15, the bottom of the temporary poolcomprises the sheet 100 and the two endless belts 15. With such anarrangement, the developer liquid is kept on the running sheetduring thepassage through the developing unit.

As is illustrated in FIG. 2, the axis of each carrier roller mayprotrude at one end out of the side board to provide a driving mechanismsuch as gear or chain which is driven by a motor.

To prevent developer leakage along the surface of the protruded axis, ashielding disk 30 is set on the axis.

Since the electrode rollers are held in such a manner that they can movefreely in the vertical direction, the side boards are provided withapertures larger than the size of the roller axis. To prevent the liquiddeveloper.

from leaking through these apertures, covers 31 and 32 are installedparallel to the direction of travel of the sheet. The electrode rollerhas a uniform diameter to form the side wall transverse to the directionof travel of the sheet for thetemporary pool, preventing developerleakage. Since the rollers are rotated by the friction with the imagesurface of the sheet, the friction imposed onthe rollerby the sideboards must be as light as possible. Experiments have proven that aclearance of about one-fifth mm is permissible between the side boardand the end of the roller to effect pool formation. Such clearance isalso desirable in view of frictional load for roller rotation. As isshown in FIG. 1, when a sheet is fed into the developing unit, thedevelopment electrode rollers are lifted up by the thickness of thesheet and convey the sheet, together with the carrier rollers, at thesame time forming a developer pool between adjacent roller pairs.

Suitable materials for the endless belt include electrically conductivebelts, insulating plastics, fabrics, etc. When the belt is insulating,the sheet may be connected to ground through the carrier rollers. In thecase where grounding is required through the endless belt, an insulatingplastic belt coiled with a fine metal wire may be employed, far easierto make than a metallic belt.

The image surface of an electrophotographic sheet.

has microscopic roughness in the surface. Where the degree of roughnessis high, one may bring the electrode rollers into direct contact withthe image surface during development without causing deterioration ofthe developed image clue to directly contacting the roller, since theroller surface is only in virtual contact with the minutely raisedportions of the sheet surface. In such case, as a matter of course, thedirectly contacted raised portions are soiledby toner. Such soiling,however, is hardly recognizable by the naked eye as long as thedimensions of the raised portions are less than that which is resolvableby human eyesight (0.1 mm or less).

The apparatus as shown in FIGS. 1 and 2 is used only for roughenedsheets. If, on the contrary, the roughness is small, that is, the imagesurface is quite smooth, the

FIG. 5 shows a schematic sectional side view of the I other embodiment.In this apparatus, the endless belts 15 are suspended on both side tipsof carrier rollers 2-6. The difference between the apparatus of FIGS. 3and 5 is that in the latter apparatus the endless belts 15 are notsuspended on the end rollers 1 and 7. This apparatus is divided by sevenpairs of rollers into six divisions.

The leakage of developer through the gap becomes sufficiently small toform a developer pool when this gap is adjusted smaller than 0.5 mm.More strict limitation is required for the first roller, 8, and theterminal rollers 14 shown in FIG. 3 to prevent counterflow and leakageof developer. For these rollers a gap about 0.1 mm is permissible,whereby the pool can be formed as quickly as in the case where no gapexists between the electrode rollers and the sheet surface. Providinggrooves on the electrode roller enables ready adjustment of theelectrode gap as well as prevention of undesirable shift of the endlessbelts.

The endless belt may be made of conductive or insulating materials.Through the use of wire-coiled belts, grounding of the conductivecoating of the sheet material or an application of bias voltage may beperformed.

The sheet material can be conveyed under the grasp by the upper andlower belts at its edges along the pass with certainty even if the sheettends to curl of is in a curled condition.

In the FIG. 3 embodiment which shows a schematic sectional side view ofanother embodiment, the differene from that shown in FIG. 1 is theprovision of two endless belts 25 of narrow width. at both ends of theelectrode rollers. These endless belts ride on the edge portions of theelectrophotographic sheet and the electrode rollers are kept away fromthe image surface of the sheet by the thickness of the endless belt.

In this arrangement no pool is formed for the first and last divisions,since the developer is not fed to these divisions from the nozzles andstreams therein from the adjacent divisions, falls down through theclearance between the side boards, whereby rushing out or counterflow ofdeveloper into another unit can be eliminated. Further, smallerconductive rollers can be provided between the larger rollers to enhancethe effect of electrode development. These smaller rollers are also heldby the side boards so that they are movable along the verticaldirection. These smaller'rollers work also as development electrodes,and may be conductive plates or A-shaped electrodes as shown in FIGS. 6aand 6b.

The pass of the sheet is most preferably straight, as is the case withthe illustrated embodiments, in view'of the ease with which theelectrode gap can be maintained uniform. However, to improve thedraining off of developer and prevent the flowing out of developer, thepass may be made arc-shaped.

The main electrodes may be made of flat metal plate, or a large numberof fine rollers may be closely arranged. The number of the carrierrollers may also be more or less than seven which was the number of thecarrier rollers for the illustrated embodiments.

In the present invention, the upper rollers and the lower rollers may beelectrically connected or insulated from each other, or a bias voltagemay be applied therebetween according to the choice of normal andreversal'development.

What we claim'is:

1. An electrophotographic developing apparatus comprising:

i. multiple carrier rollers, held in and between fixed side platesupporting meansyconveying an electrotween said carrier rollers and saiddevelopment electrodes, whereby a developer pool is formed on theelectrophotographic sheet, said pool being limited by the sheet and theendless belts as bottom plane, and the adjacent development electrodesand the fixed side plate supporting means as side borders, said sheetbeing conveyed with its rear surface in contact with said endless belts.

2. Apparatus as in claim 1 wherein the development electrodes arerollers and including a further pair of endless belts disposed at theends of said development rollers for spacing said development rollersfrom said electrophotographic sheet material.

1. An electrophotographic developing apparatus comprising: i. multiplecarrier rollers, held in and between fixed side plate supporting means,conveying an electrophotographic sheet material to be processed anddriven at a common speed by driving means; ii. two endless belts hung atthe side ends of said carrier rollers to form a loop including at leastsome of said carrier rollers, said endless belts being contiguous tosaid fixed side plates; iii. multiple development electrodes arrangedfacing to carrier rollers; and iv. a means of supplying liquid developeronto the electrophotographic sheet which is conveyed between saidcarrier rollers and said development electrodes, whereby a developerpool is formed on the electrophotographic sheet, said pool being limitedby the sheet and the endless belts as bottom plane, and the adjacentdevelopment electrodes and the fixed side plate supporting means as sideborders, said sheet being conveyed with its rear surface in contact withsaid endless belts.
 2. Apparatus as in claim 1 wherein the developmentelectrodes are rollers and including a further pair of endless beltsdisposed at the ends of said development rollers for spacing saiddevelopment rollers from said electrophotographic sheet material.